The invention relates to an apparatus for plasma treatment of surfaces. More particularly, the invention relates to a pretreatment enabling the coating of surfaces of synthetic resin materials with adhesives, printing inks and the like. Such a pretreatment is necessary because the surfaces of synthetic resins can normally not be wetted with liquids and do therefore not accept the printing ink or the adhesive. The pretreatment changes the surface structure of the synthetic resin so that it becomes wettable with liquids having a comparatively large surface tension. The surface tension of the liquids with which the surface can still be wetted is an indictor for the quality of the pretreatment.
A well established method for the pretreatment of synthetic resin surfaces is based on the principle of corona discharge. In this method the resin to be treated is typically passed through between two electrodes which are coated with a ceramic material and to which a high voltage with high frequency is applied so that a corona discharge occurs through the synthetic resin material. However, this method is suitable only for relatively thin workpieces having a flat surface, such as plastic foils.
DE 195 32 112 A discloses an apparatus for pretreatment of surfaces by means of a plasma jet. Thanks to a specific construction of the plasma nozzle a relatively cool but nevertheless highly reactive plasma jet is achieved which has a shape and dimensions similar to those of a flame of a candle and, as a consequence, permits also the pretreatment of profiles having relatively deep recesses. Due to the high reactivity of the plasma jet a short pretreatment time is sufficient, so that the workpiece can be passed along the plasma jet with a relatively high velocity. Thus, as a result of the comparatively low temperature of the plasma jet, a pretreatment of heat-sensitive plastic materials is also possible. Since no counter electrode is necessary on the back side of the workpiece, it is also possible to pretreat the surfaces of arbitrarily thick block-like workpieces, hollow bodies, and the like. For a uniform pretreatment of larger surfaces, the cited publication discloses an array of a plurality of staggered plasma nozzles. In this case, however, relatively high expenses are necessary for the equipment.